“When I was visualizing the scene, it didn’t look quite as heroic as I intended it to be. So then I put the Poppins line in Peter Quill’s mouth. And Yondu, of course, not knowing the name Mary or Poppins, might think he’s sort of a Lee Marvin type, and he owns it.” – James Gunn
Mary Poppins in the original books is implied to be an old God who makes every other fantastical creature SHIT THEIR PANTS at the mere mention. Mary Poppins is much cooler than you think.